Dielectric mixture composition linear sensor with compensation for mixture electrical conductivity

ABSTRACT

The composition of an electrically conductive mixture, preferably a mixture of gasoline and alcohol, is measured using a single measurement cell having a first electrode and a second electrode with a measurement space therebetween for receiving a specimen of the mixture. With a specimen of the mixture in the measurement space, the first electrode is alternatively connected to a reference discharge voltage, preferably ground, and to a feedback control voltage. A first operational state encompasses the period when the first electrode is connected to the reference discharge voltage and a second operational state encompasses the period when the first electrode is connected to the feedback control voltage. A first operational state peak voltage is measured during the first operational state, and a peak second operational state voltage is measured during the second operational state. The first operational state voltage and a preselected constant proper fraction of the peak second operational state voltage are mathematically combined to obtain a voltage output, which may be correlated to the composition of the mixture. The voltage output is also used as the feedback control voltage, to produce a linear signal output as a function of dielectric constant of the conductive mixture.

BACKGROUND OF THE INVENTION

This invention relates to the determination of the composition ofelectrically conductive mixtures, and, more particularly, to thedetermination of the composition of a gasoline-alcohol mixture using acompensated dielectric measurement.

It is sometimes necessary to determine the composition of a mixturewhich is electrically conductive because one or more of the componentsof the mixture are electrically conductive. One of the most important ofsuch situations is the determination of the composition of agasoline-alcohol mixture used as the fuel in the internal combustionengine of an automotive vehicle. In some areas, alcohol is added togasoline for economic and environmental reasons. It is necessary to varythe settings of the engine that uses such a fuel responsive to thealcohol content and composition of the mixture, in order to ensure itsclean, efficient operation.

To control the engine settings responsive to the composition of thefuel, it is first necessary to measure the composition of the fuel in areliable fashion. Techniques and apparatus for making such measurementsof automotive fuels in real time are known. U.S. Pat. Nos. 5,231,258,5,103,184, and 5,089,703 disclose a fuel sensor that makes suchmeasurements capacitively. This approach is operable and reasonablyaccurate in many situations.

However, a potential inaccuracy in the composition measurements arisesbecause the fuel may contain impurities such as salts that causevariations in its electrical conductivity even at constant composition.The dielectric properties of the fuel, upon which the capacitancemeasurements are based, vary somewhat according to its electricalconductivity. In particular, the actual composition of a highlyconductive gasoline-alcohol fuel mixture may be somewhat different fromthat reported from readings of the capacitive sensor, because of suchelectrical conductivity effects.

In one approach to overcoming this problem, two sensors are provided,one to measure the capacitance of the fuel and the other to measure theelectrical conductivity of the fuel. This approach is expensive in thattwo separate sensors are required, and may lead to instrumentation-basedinaccuracies due to changes in characteristics of the two sensors overtime.

There is a need for an improved approach to measuring the composition ofa mixture such as a gasoline-alcohol mixture, which is both inexpensiveto perform and accurate. The present invention fulfills this need, andfurther provides related advantages.

SUMMARY OF THE INVENTION

The present invention provides an apparatus and method for determiningthe composition of an electrically conductive mixture such as agasoline-alcohol mixture. A more accurate determination of mixturecomposition is obtained than previously possible, with little addedcost. The approach is based upon a capacitive measurement of the typealready known in the art, but where the result of the capacitivemeasurement is corrected for the electrical conductivity of the mixture.A single measurement cell of the type already known is used, andadditional measurement capability is provided for more completelyutilizing the data obtained during the capacitive measurement. Theoutput signal of the apparatus is linear with dielectric constant of theconductive mixture and may be correlated with the composition of themixture. Additionally, the sensitivity of the measurement is improved.Only minimal additional cost to obtain this additional accuracy andsensitivity is required.

In accordance with the invention, an apparatus for measuring thecomposition of an electrically conductive mixture comprises ameasurement cell including a first electrode and a second electrode witha measurement space therebetween for receiving a specimen of anelectrically conductive mixture. The second electrode is connected to afixed second electrode potential, preferably ground. A switch has afirst pole in electrical communication with the first electrode and afeedback control voltage through a resistor, and a second pole inelectrical communication with a reference discharge voltage, preferablyground. There is a first operational state encompassing the period whenthe switch is closed and a second operational state encompassing theperiod when the switch is open. A first measurement circuit element hasan input in electrical communication with the first electrode and anoutput of a first operational state peak voltage measured during thefirst operational state. A second measurement circuit element has aninput in electrical communication with the first electrode and an outputof a peak second operational state voltage measured during the secondoperational state. A mathematical circuit element has as inputs thefirst operational state peak voltage and the second operational statepeak voltage, and as a voltage output a mathematical function of theinputs. The voltage output is supplied to the switch as the feedbackcontrol voltage. A correlator may be used to associate the summedvoltage with the composition of the mixture.

Preferably, the mathematical circuit element is a summing circuitelement that sums the first operational state voltage and a preselectedconstant proper fraction of the peak second operational state voltage.The constant fraction is typically from about 0.1 to about 0.2, and ismost preferably about 0.15.

In a related aspect, a method for measuring the composition of anelectrically conductive mixture comprises the steps of providing ameasurement cell including a first electrode and a second electrode witha measurement space therebetween for receiving a specimen of a mixture,the second electrode being connected to a fixed second electrodepotential, and placing a specimen of an electrically conductive mixturein the measurement space. The method further includes alternativelyconnecting the first electrode to ground and to a feedback controlvoltage. A first operational state encompasses the period when the firstelectrode is connected to ground, and a second operational stateencompasses the period when the first electrode is connected to theapplied voltage. A first operational state peak voltage is measuredduring the first operational state, and a second operational state peakvoltage is measured during the second operational state. The firstoperational state voltage and the peak second operational state voltageare mathematically combined to obtain a voltage output. The voltageoutput is provided as the feedback control voltage. The voltage outputmay be correlated with the composition of the mixture.

The peak voltage obtained during the first operational state, typicallya negative peak voltage, reflects a capacitive measurement of themixture, and specifically its dielectric constant upon which the firstoperational state peak voltage depends. This value, however, dependsupon the electrical conductivity of the mixture, and may be slightlyerroneous when the mixture has a high electrical conductivity. The peakvoltage obtained during the second operational state, typically apositive peak voltage, reflects an electrical conductivity (oralternatively stated, resistivity) measurement of the mixture. Thissecond operational state peak voltage is of opposite sign from that ofthe first operational state peak voltage. Adding together the firstoperational state peak voltage and a fixed proper fraction of the peaksecond operational state voltage yields a summed voltage output that hasbeen found to correlate closely with the actual mixture composition overa wide range of mixture electrical conductivities and mixturegasoline-to-alcohol ratios, and in particular for high mixtureelectrical conductivities where inaccuracies arose in priormeasurements.

Linearization of the output is obtained by using the summed voltageoutput, which is a linear function of the capacitance of the mixture inthe measurement cell, as the feedback control voltage. The sensitivityof the measurement is also improved.

The improved accuracy, sensitivity, and linear output are obtainedwithout the need for a second sensor, and with only measurementmodifications to a well-proved capacitive measurement sensor. Theadditional cost to obtain significant additional accuracy is thereforeminimal. Other features and advantages of the present invention will beapparent from the following more detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings, whichillustrate, by way of example, the principles of the invention. Thescope of the invention is not, however, limited to this preferredembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective cutaway view of a motor vehicle having an enginefuel control with a fuel composition sensor according to the presentinvention;

FIG. 2 is a side sectional view of a preferred fuel sensor;

FIG. 3 is a sectional view of the fuel sensor of FIG. 2, taken generallyalong lines 3--3;

FIG. 4 is a sectional view of the fuel sensor of FIG. 2, taken generallyalong lines 4--4;

FIG. 5 is a schematic view of the electronics circuitry of the sensor;

FIG. 6 is a graph of voltage as a function of time at the measurementpoint of the electronics circuitry;

FIG. 7 is a schematic view of a preferred implementation of theelectronics circuitry of the sensor; and

FIG. 8 is a electronic schematic diagram of a preferred positive peakdetector

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 depicts a motor vehicle 10 with an internal combustion engine 11in an engine compartment 12. The engine 11 receives fuel from a fueltank 13 at the opposite end of the vehicle through a fuel conduit 15 andreturns excess fuel to the tank 13 through a fuel conduit 14. The fuelin tank 13 may be a mixture of two fuels, one of which is gasoline andthe other of which is an alcohol such as methanol or ethanol. The fuelmay additionally include small impurity amounts of salts and water, orother electrically conductive compounds. The relative amounts of themajor components, gasoline and alcohol, and the amount, if any, of anyimpurities present, such as salts and water, are not known a priori.

The engine is controlled responsive to the relative amounts of thegasoline and the alcohol present. To obtain information as to therelative amounts of gasoline and alcohol present in the fuel, the fuelconduit 15 includes therein a fuel composition sensor 16 located withinthe engine compartment 12 at a point close to the engine 11. The fuelcomposition sensor 16 generates an output signal indicative of therelative fractions of gasoline and alcohol in the fuel flowingtherethrough.

A standard fuel vapor collection canister 17 is connected by a vaporconduit 18 to the fuel tank 13 for collection of vapor therefrom.Another vapor conduit 19 extends from the canister 17 to the inductionsystem of the engine 11 to deliver the collected vapor to the engine 11for combustion.

The operation of the engine 11 is controlled by an electronic controller20, which may be located at the rear of the engine compartment as shownor at any other convenient location. The controller 20 may be aprogrammed digital computer similar to those presently used in motorvehicles for engine control. The apparatus is well known, comprising amicroprocessor, RAM, ROM, and appropriate input/output circuitry, withan appropriate program stored in ROM to coordinate receipt of inputinformation from various sensors, including the sensor 16, performcalculations and table look-ups, and output commands to variousactuators of engine-related components. The controller 20 is responsiveto the fuel composition output signal from the fuel composition sensor16 as well as to a fuel temperature signal therefrom to modify suchengine operating parameters as the air/fuel ratio, ignition timing,canister purge rate, and/or other parameters as necessary to optimizeengine operation for the actual fuel mixture provided to the engine assensed by the sensor 16.

The mechanical structure of a preferred version of the sensor 16 isillustrated in FIGS. 2-4 and is also described in U.S. Pat. No.5,103,184, whose disclosure is incorporated by reference. This structureprovides for a common ground between the sensor case and the outercapacitor electrode, so that it is not necessary for these parts to beinsulated from each other, although an insulated approach without acommon ground is also operable.

An outer tube 26 of low carbon stainless steel has a first end fittedwithin and laser welded to an extended open end of a coupling member 21and a second end fitted within an laser welded to an extended open endof another coupling member 22. The coupling members 21 and 22 are bothmade of stainless steel, have an outer hexagonal shape, and definepassages 23 and 24, respectively, therethrough. Each of the passages 23and 24 is provided with an internally threaded portion 27 and 28,respectively, for attachment with an appropriate fitting to the fuelline 15. The outer tube 26 forms a portion of the outer case of thesensor 16 as well as the outer electrode of a sensor capacitor.

A stainless steel inner tube 40 is disposed axially within the outertube 26 to form the inner electrode of the sensor capacitor and todefine an annular chamber 25 between the tubes 26 and 40. The inner tube40 is closed at each end in a tapered portion 41, a smaller diametercylindrical portion 42, and an extended axial nipple 43, all made ofstainless steel. The inner tube 40 is suspended at each end thereofwithin the outer tube 40 by spacers 30, seen axially in FIG. 3. Eachspacer 30 includes an inner portion 31 having a circular opening 32which receives the cylindrical portion 42 of the inner tube 40 and threeradially projecting legs 33 at 120 degree angles with respect to eachother, which end at the outer tube 26. The legs 33 define openings 34therebetween. The spacers 30 may be made of an alcohol resistant,electrically insulating polymeric resin such as Nylon^(R). A fuel flowpath is thus defined through passage 24 of coupling member 22, throughthe openings 34 between the legs 33 of the first spacer 30, through theannular chamber 25 between the tubes 26 and 40, through the openings 34between the legs 33 of the second spacer 30 at the other end of thetube, and through the passage 23 of the coupling member 21. The fuel inthe chamber 25 between tubes 26 and 40 forms the dielectric of thesensor capacitor defined by the tubes 26 and 40, which serve as theelectrodes.

A circuit board 36 of standard construction is mounted as part of thesensor 16 and supports the electronic circuitry discussed subsequently.The circuit board 36 is attached to coupling member 21 by a machinescrew 37. The machine screw 37 not only physically anchors one end ofthe board 36 but also provides an electrical ground connection between aground circuit trace on the circuit board 36 and, through couplingmember 21, the tube 26. The tube 26 thus comprises a grounded electrodeof the sensor capacitor. Between screw 37 and the adjacent spacer 30, aglass insulator 38 is retained in an opening of the coupling member 21adjacent to the circuit board 36. A stainless steel pin 39 projectsthrough the circuit board 36 and the insulator 38 so that it does notcontact the coupling member 21 or the tube 26. The pin 39 is maintainedin physical and electrical contact, as by welding or soldering, to theadjacent nipple 43 and is soldered on circuit board 36 to theappropriate trace for the inner electrode of the sensor capacitor. Theinner and outer electrodes are thus electrically coupled to theappropriate elements on the circuit board.

The opposite end of the circuit board 36 is attached by a pair of screws44 to a clamp 45 comprising upper and lower clamp members 46 and 47,respectively. The screws 44 hold the clamp members 46 and 47 together aswell as securing the clamp 45 to the circuit board 36. The clamp members46 and 47 include matching semicircular openings which, together, definean inner circular opening 48 for receiving the outer tube 26. The clamp45 fits snugly around the tube 26 to hold the opposite end of thecircuit board 36 in a stable manner without undue vibration relative tothe tube 26 but which allows relative rotation so that twisting torquesapplied between coupling members 21 and 22 are not applied to thecircuit board 36. In practice, to protect the components of the circuitboard 36 and other parts of the sensor 16 from the environment, asupplemental external case, not shown, may be attached to the sensor 16as by a machine screw in a threaded depression, not shown, opposite thescrew 37 and at clamp 45, for example by extending the bolts 44completely therethrough to surround all of the sensor except thehexagonal ends of the couplings 21 and 22.

A fuel temperature sensor 50 is received in a sealing manner in the wallof the coupling member 22 so as to be exposed to fuel within. The sensor50 generates a fuel temperature signal communicated to an appropriatecircuit trace on the circuit board 36 through wire leads 51 and 52. Thefuel temperature sensor may be a thermistor or any other type oftemperature sensor appropriate for sensing the temperature of fluids andprovides a temperature correction signal for the sensor output. The fueltemperature signal may be separately output to the engine controlcomputer 20 for temperature compensation of the fuel composition signalreceived from the sensor 16.

Although all openings and gaps within the sensor 16 are of sufficientsize to prevent them from presenting a significant restriction to fuelflow, the total volume of fuel contained within the chamber 25 isminimized to prevent extreme reduction or accumulation of fuel withinthe chamber 25 which might lead to differences in dielectric constantbetween the fuel mixture in the sensor and that about to enter thecombustion chamber of the engine 11.

FIG. 5 schematically depicts a sensor electronics circuit 100 inrelation to the fuel sensor 16. The components of the circuit 100 arepreferably mounted on the circuit board 36 and connected to the interiorcomponents of the sensor 16 in the manner previously described. Thecomponents may be provided as discrete components for testing or as anintegrated circuit for mass production.

In the circuit 100, an oscillating switch 102 alternating connects afirst side 104 of a reference capacitor 106, C_(ref), to a feedbackcontrol voltage 108, V_(fc), through a resistor R, and to ground 110. Asecond side 112 of the reference capacitor 106 is connected to ground114 through a variable capacitor 116 formed by the tubes 24, 40 and thefuel in the chamber 25 of the sensor 16. As illustrated, the inner tube40 is connected to the second side 112 of the reference capacitor 106.The outer tube 26 is connected to ground 114. The fuel to be sensedflows in the chamber 25 defined between the tubes 26 and 40 and formsthe dielectric medium of the variable capacitor 116.

The present approach measures voltage responses at a measurement point120 at the second side 112 of the capacitor 106, or equivalently, theinner tube 40. Starting from an initial condition with the switch 102open, when the switch 102 is closed after the reference capacitor 106has been previously charged, the reference capacitor 106 discharges toground 110 and a transient negative voltage spike 122 having a maximum(negative) peak value 124 is observed at the measurement point 120, asseen in FIG. 6. (The transient voltages discussed herein presuppose thatV_(fc) is positive relative to ground. They are reversed if V_(fc) isnegative.) When the switch 102 is later opened, the reference capacitor106 charges from the reference voltage 108. A transient positive voltagespike 126 with a maximum (positive) peak value 128 is observed.

These same transient voltages were observed and reported at col. 7,lines 29-39 of the '184 patent, see also FIG. 7(C) of the '184 patent.The '184 patent makes use of the information in the negative voltagespike 122, but expressly states that it does not use the information inthe positive voltage spike 126.

The present inventors have discovered that the information in thepositive voltage spike 126 may be used to correct the output informationfor the conductivity variations in the fuel. Accordingly, the circuit100 is provided with a first measurement circuit in the form of anegative peak detector 130 to detect the negative peak value 124(V_(peak-)), and a second measurement circuit in the form of a positivepeak detector 132 to detect the positive peak value 128 (V_(peak+)). Thenegative peak detector 130 is preferably a comparator/pulse integratorfeedback circuit, and the positive peak detector 132 is preferably anoperational amplifier/comparator circuit.

The outputs of the peak detectors 130 and 132 are provided to a summingcircuit element in the form of an adder 134. The adder 134 addsmathematical functions of the negative peak value 124 and the positivepeak value 128 together. The output signal of the adder 134 is providedto the engine controller 20 as the output signal of the sensor 16. Moregenerally, the negative peak value 124 and the positive peak value 128may be combined in a nonlinear fashion by a different circuit elementsubstituted for the element 134. The inventors have found that a linearcombination yields a good correlation with the test data, but in othercircumstances nonlinear combinations of the inputs may be required tocorrelate the test data.

For the sensor geometry of FIGS. 2-4, the preferred mathematicalfunction of the negative peak value V_(peak-) is the negative peak valueitself. The preferred mathematical function of the positive peak valueV_(peak) ₊ is a preselected constant proper fraction of the positivepeak value. The constant proper fraction ranges from about 0.1 to about0.2. Correlations have demonstrated that selection of a proper fractionof 0.15, in conjunction with the voltage feedback of V_(fc) to bediscussed subsequently, reduces errors caused by conductive componentsin the fuel to less than one percent over a wide range of fuelcomposition (fractions of gasoline and alcohol, for both ethyl andmethyl alcohols) and conductive impurity contents. Thus, the preferredoutput of the integrator 134 for the sensor of FIGS. 2-4 is (V_(peak-)+0.15 V_(peak+)).

The output voltage V_(out) is also provided as the feedback controlvoltage V_(fc). It has been determined that V_(out) is a linear functionof the capacitance of the mixture in the measurement cell.

As discussed in the '184 patent, the value of V_(peak-) gives anindication and measurement of the dielectric constant of the fuel in thechamber 25. However, studies by the present inventors have determinedthat the dielectric constant value determined from V_(peak-) is alsoweakly a function of the fuel electrical conductivity (or equivalently,resistivity). Consequently, there may be an error in the determinationof the fuel composition if only the value of V_(peak-) is used toestablish the output signal of the sensor 16, as has been the priorpractice.

The studies by the inventors have further determined that the value ofV_(peak) ₊ is a function of the fuel conductivity and, also, weakly afunction of the dielectric constant. The mathematical combination of thevalues of V_(peak-) and V_(peak) ₊ may therefore be used, as describedherein, to negate the effect of the dependence of V_(peak-) on the fuelconductivity.

The components of the circuit 100 may be any operable circuit elements.However, an important consideration in the competitive world ofautomotive manufacture is obtaining satisfactory performance withminimal cost of components and production. The considerations ofsatisfactory performance and cost together determined the selection ofthe preferred circuit components for the present application.

FIG. 7 depicts a preferred form of a measurement circuit for practicalapplications with minimal cost. In FIG. 7, elements corresponding tothose of the embodiment of FIG. 5 are assigned the same referencenumerals, and the earlier description is incorporated here.

In the measurement circuit of FIG. 7, resistor 152, R_(p1), is in serieswith the reference voltage 108, to limit the current that may be drawn.A resistor 154, R_(pad), is added in parallel with the fuel sensor 16 toaid in minimizing the range of nonlinear variation.

The peak negative peak detector 130 is implemented as a high-speedcomparator 158 and a latch 160 that sequentially determine the values ofV_(peak-) and supply the values to an integrator 162. The latch 160 isprovided a clock input 164 that corresponds to a clock input 164' thattimes the switch 102, so as to capture the value of V_(peak-) properly.

The positive peak detector 132 may be either an open-loop type, with adiode or emitter-follower, or a closed loop type. The open-loop typeshave low cost and good speed, but relatively low precision. Theopen-loop types have good precision, but higher cost. An LT1016comparator of the type discussed in the '184 patent and used for thenegative peak detector is operable, but of relatively high cost.

The preferred positive peak detector 132 is illustrated in greaterdetail in FIG. 8. An input voltage from the fuel sensor 16 is providedto a differential amplifier 180, which receives a current input from atemperature-compensated current source 182. If the positive peak voltageis less than the input voltage, the differential amplifier 180 turns ontransistor 184. Capacitor 186 then charges rapidly until the outputvoltage equals the input voltage. Transistor Q2 of the differentialamplifier 180 is thence turned on, transistor Q1 is turned off, andtransistor 184 is turned off so that the output and input voltages arethe same. The peak voltage value is followed by the emitter-followerbuffer 188. As the input voltage drops as shown in FIG. 6, the outputvoltage V_(peak) ₊ is maintained due to the discharge of the capacitor186 through a resistor 190 over a relatively long period of time. Thispositive peak detector is relatively inexpensive and is operable tocapture the peak voltage which has a rise time in the 1-10 microsecondrange. This relatively slow response is actually desirable, as itignores transients on the signal. The accuracy of this circuit is notparticularly great, but it is sufficient in view of the relatively weakdependence of the result on the value of V_(peak) ₊.

The peak values determined by the negative peak detector 130 and thepositive peak detector 132 are supplied to the adder 134 or otherelement for mathematically combining the positive and negative peakvalues, as discussed previously. The output of the adder 134, V_(out),is supplied to the engine controller. It is also supplied as V_(fc) tothe switch 102 and the first side 104 of the capacitor C_(ref) 106.

For some applications, it may be desirable to provide a frequency outputrather than a voltage output. For such situations, the voltage outputV_(out) of the adder 134 is supplied to a voltage-to-frequency converter200, resulting in a signal f_(out) whose frequency is proportional toV_(out). Optionally, temperature information from the fuel temperaturesensor 50 may be encoded as an amplitude modulation onto the frequencysignal f_(out) by the voltage-to-frequency converter 200.

The V_(out) or f_(out) signals may be used directly for controlfunctions, or they may optionally be correlated with prior test data toobtain the actual values of the composition of the mixture. Correlationis preferably accomplished using a look-up table in the electroniccontroller 20.

The circuit of FIG. 7 may be selected to have a set of most-preferredcomponents and values, as discussed next. The fuel sensor 16 ispreferably the cylindrical geometry of FIGS. 2-4, with the outer radiusof the inner tube 40 being 0.2275 inches and the inner radius of theouter tube 26 being 0.2275. The voltage V_(ref) is +3.5 volts DC. Theswitch 102 is preferably a 2N7000 MOSFET driven at a switching speed ofabout 5 kilohertz. The value of R_(p1) is chosen as a compromise betweenincreasing the value of R_(p1) to decrease the dependence of V_(peak) ₊on the capacitance and decreasing the value of R_(p1), which increasesthe value of V_(peak+), making it easier to measure. For the preferredcase, Rp1 was chosen as 5,000 ohms. R_(pad) was chosen as 2000 ohms.

A fuel sensor system was constructed according to the preferred approachdescribed herein. Measurements have been made with this measurementapproach as compared with the prior approach. When only the negativepeak voltage V_(peak-) is used to determine the mixture composition, asin the prior approach, there is typically about a 10 percent variationin the determined mixture composition as compared with the actualmixture composition. That is, if the actual mixture composition is 20percent alcohol, the measured mixture composition may be as low as 18percent or as high as 22 percent. When the preferred embodiment of thepresent approach using both V_(peak-) and V_(peak) ₊ is used, thevariation is reduced to about 1 percent variation, a significantimprovement for the control of the engine. Additionally, the use of thefeedback voltage V_(fc) improves the sensitivity of the presentmeasurement approach as compared with the prior approach and an approachlike the present approach except where there is no feedback signal.Consequently, the full-scale voltage output is greater with the presentapproach, making instrumentation and control easier.

Although a particular embodiment of the invention has been described indetail for purposes of illustration, various modifications andenhancements may be made without departing from the spirit and scope ofthe invention. Accordingly, the invention is not to be limited except asby the appended claims.

What is claimed is:
 1. Apparatus for measuring the composition of anelectrically conductive mixture, comprisinga measurement cell includinga first electrode and a second electrode with a measurement spacetherebetween, the second electrode being connected to a fixed secondelectrode potential; a switch having a first pole in electricalcommunication with the first electrode and a feedback control voltagethrough a resistor, and a second pole in electrical communication with areference discharge voltage, there being a first operational stateencompassing the period when the switch is closed and a secondoperational state encompassing the period when the switch is open; afirst measurement circuit element having an input in electricalcommunication with the first electrode and an output of a firstoperational state peak voltage measured during the first operationalstate; a second measurement circuit element having an input inelectrical communication with the first electrode and an output of asecond operational state peak voltage measured during the secondoperational state; and a mathematical circuit element having as inputsthe first operational state peak voltage and the second operationalstate peak voltage, and as a voltage output a mathematical function ofthe inputs, the voltage output further serving as the feedback controlvoltage.
 2. The apparatus of claim 1, wherein there is no paddingcapacitor connected between the first electrode and the referencedischarge voltage.
 3. The apparatus of claim 1, further includingavoltage-to-frequency converter having as an input the summed voltage andas an output a signal whose frequency varies responsive to the summedvoltage.
 4. The apparatus of claim 1, wherein the measurement cellfurther includesa temperature sensor in thermal communication with themeasurement space, the temperature sensor having as an output thetemperature of the measurement space.
 5. The apparatus of claim 4,further includinga voltage-to-frequency converter having as a firstinput the summed voltage and as a second input the output of thetemperature sensor, and as an output a signal whose frequency variesresponsive to the summed voltage and which is modulated responsive tothe output of the temperature sensor.
 6. The apparatus of claim 1,wherein the feedback control voltage is positive relative to thereference discharge voltage, the first operational state peak voltage isnegative, and the peak second operational state voltage is positive. 7.The apparatus of claim 1, wherein the measurement cell is generallycylindrically symmetric and each of the electrodes is cylindrical, oneof the electrodes having a smaller cylindrical diameter than the otherelectrode and being disposed inside and in facing relationship to theother electrode.
 8. The apparatus of claim 1, wherein the switch is atransistor.
 9. The apparatus of claim 1, wherein the first measurementcircuit element is a comparator/pulse integrator feedback circuit. 10.The apparatus of claim 1, wherein the second measurement circuit elementis an operational amplifier/comparator circuit.
 11. The apparatus ofclaim 1, wherein the mathematical circuit element comprisesa summingcircuit having as inputs the first operational state peak voltage andthe peak second operational state voltage, and as a summed voltageoutput the sum of the first operational state voltage and a preselectedconstant proper fraction of the peak second operational state voltage.12. The apparatus of claim 11, wherein the preselected constant properfraction is from about 0.1 to about 0.2.
 13. The apparatus of claim 11,wherein the preselected constant proper fraction is about 0.15.
 14. Theapparatus of claim 1, wherein the switch operates at a frequency of fromabout 5 to about 10 kilohertz.
 15. The apparatus of claim 1, furtherincludinga correlator comprising an associative relation between thesummed voltage output and the composition of a mixture.
 16. Theapparatus of claim 1, wherein the reference discharge voltage is ground.17. The apparatus of claim 1, wherein the fixed second electrodepotential is ground.
 18. Apparatus for measuring the composition of anelectrically conductive mixture, comprisinga measurement cell includinga first electrode and a second electrode with a measurement spacetherebetween for receiving a specimen of an electrically conductivemixture, the second electrode being connected to ground; a switch inelectrical communication with the first electrode, the switch beingoperable to alternatively connect the first electrode to ground and to afeedback control voltage through a resistor, there being a firstoperational state encompassing the period when the first electrode isconnected to ground and a second operational state encompassing theperiod when the first electrode is connected to the feedback controlvoltage; a first measurement circuit element having an input inelectrical communication with the first electrode and an output of afirst operational state peak voltage measured during the firstoperational state; a second measurement circuit element having an inputin electrical communication with the first electrode and an output of apeak second operational state voltage measured during the secondoperational state; and a summing circuit element having as inputs thefirst operational state peak voltage and the peak second operationalstate voltage, and as a summed voltage output the sum of the firstoperational state voltage and a preselected constant proper fraction ofthe peak second operational state voltage, the summed voltage beingsupplied to the switch as the feedback control voltage.
 19. A method formeasuring the composition of an electrically conductive mixture,comprising the steps ofproviding a measurement cell including a firstelectrode and a second electrode with a measurement space therebetween,the second electrode being connected to a fixed second electrodepotential; placing a specimen of an electrically conductive mixture inthe measurement space; alternatively connecting the first electrode toground and to a feedback control voltage through a resistor, a firstoperational state encompassing the period when the first electrode isconnected to ground and a second operational state encompassing theperiod when the first electrode is connected to the feedback controlvoltage; measuring a first operational state peak voltage during thefirst operational state; measuring a peak second operational statevoltage during the second operational state; mathematically combiningthe first operational state peak voltage and the second operationalstate peak voltage to obtain a voltage output; and providing the voltageoutput as the feedback control voltage.
 20. The method of claim 19,wherein the step of placing includes the step ofproviding a mixture ofgasoline and alcohol.
 21. The method of claim 19, including anadditional step, after the step of summing, ofcorrelating the voltageoutput with the composition of the mixture.
 22. The method of claim 19,wherein the step of alternatively connecting includes the stepofproviding a feedback control voltage that is positive relative to thefixed second electrode potential, and wherein the fixed second electrodepotential is ground.
 23. The method of claim 19, wherein the step ofmathematically combining connecting includes the step ofsumming thefirst operational state voltage and a preselected constant properfraction of the peak second operational state voltage to obtain thevoltage output.